Pedology

1. Pedology • Soil morphology: color, physical structure, and chemical and mineralogical properties of horizons • Soil genesis: processes of soil formation • Soil classification: categorization of soils in groups according to their morphological properties and/or assumed genesis • Soil survey: determination of type and pattern of the occurrence of soil bodies on the landscape • Soil characterization: determination and quantification of chemical, physical, mineralogical, and biological properties of samples collected from soil horizons. • Soil interpretation: analysis of soil data to infer suitability, potential use, and limitations for various uses.

2. Concepts and Definitions of Soil • Soil has been defined by many groups in a variety of ways. • Geology • Engineering • Plant productivity • “Natural body” • Pedological definition - the collection of natural bodies on the earth's surface containing living matter and supporting or capable of supporting plants out-of-doors. "each soil has a unique morphology resulting from the combination of climate, living matter, and relief acting upon earthy parent materials over time."

3. Miller’s definition “ weathered material covering the earth’s surface that has been affected by specific soil-forming processes” --transformations --additions, losses --translocation

6. Concepts and Definitions of Soil • The upper limit of soil is the boundary between soil and air, shallow water, live plants, or plant materials that have not begun to decompose. • The horizontal boundaries of soil are areas where the soil grades to water too deep to support rooted plants, barren areas, rock, or ice. • The lower boundary of soil is most difficult to define. • Soil consists of the horizons have been altered by the interactions of climate, relief, and living organisms over time. • Hard rock or earthy materials devoid of biological activity are not soil. • Lower limit of biologic activity is gradual and difficult to detect. • For practical and classification purposes, the lower boundary of soil is arbitrarily set at 200 cm.

7. Concepts of Soil Genesis • Pedogenic processes active today have been operating over time and have varying degrees of expression over space. • Many soil-forming processes proceed simultaneously, and the properties of the resulting soil are the result of the balance among the processes. • Distinctive processes produce distinctive soils. • Five environmental factors, climate, organisms, relief or topography, parent material, and time, mediate the pedogenic processes. • Current soils carry the imprint of a combination of pedogenic processes that have been active over the period of soil development. • A particular site may have had many different soils as one or more of the factors influencing soil formation changed over time (paleosoils). • There are few (very) old soils.

8. A Few Definitions • Regolith: • Pedon: • Soil profile: • Horizon: • Solum:

9. Soil Properties Describing soil properties in the field: Color Texture Structure Consistence and other stuff….

10. Soil Color

13. Soil Color • Obvious property and commonly described • "black soils" • "red soils" • "brown soils” • It can be used to infer • minerals present in the soil • stage of development, and • seasonal saturation. • Color also may affect a soil's classification

14. Soil Coloring Agents • Uncoated mineral grains • Any color • Most commonly white to gray • Coatings on grains • Color depends on composition • Organic matter - brown to black • Subsoil horizon color is color of Fe and Mn oxides and oxyhydroxides coating grains. • Hematite (Fe2O3) - red • Geothite (-FeOOH) – yellowish brown • Lepidocrocite (-FeOOH) - orange to yellowish orange • Mn oxides – black

15. Soil Color Determination • The Munsell Color System • The Munsell system has three components • Hue - (page) measure of chromatic composition of light reaching the eye. • Value - (vertical scale) degree of lightness or darkness of a color. • Chroma - (horizontal scale) relative purity or strength of the spectral color. • The Munsell notation is written symbolically as H V/C • 2.5YR 6/8. • The notation for a neutral color is written: N V/; N 5/

17. Munsell Color Space

19. Mottles & Redoximorphic Features • Horizons with multiple colors • The dominant color is the matrix color • Minor colors are either mottles or redoximorphic features • Mottles - areas of different color due to processes other than reduction and oxidation of Fe and Mn • Does not include coatings or rock fragments • Redoximorphic Features - features that have resulted from reduction, oxidation, and movement of Fe and Mn associated with seasonal saturation. • Both described by abundance, size, contrast, and color • Other characteristics can be described if they are important or diagnostic. • shape • boundary • location • composition

20. Redoximorphic Features • Features whose color is the result is reduction, oxidation, and movement of Fe and Mn because of seasonal saturation • Redox depletions - areas with lower chroma than the matrix caused by depletion (loss) of Fe and Mn • Redox concentrations - areas with higher chroma and/or redder hue than the matrix caused by concentration of Fe and Mn • Mobility of Fe and Mn in soils is related to redox processes caused by saturation with water.

23. Redox Feature Formation • Fe and Mn oxides and oxyhydroxides (Fe2O3, FeOOH; MnO2, MnOOH; Fe3+ and Mn4+) are very sparingly soluble in water and immobile in soils. • Reduction of Fe and Mn (Fe2+ and Mn2+) changes mineral form and results in compounds that are soluble in water. • Dissolution of these compounds releases Fe2+ and Mn2+ into solution and they can move by mass flow and diffusion.

24. Reduction • O2 is used as the electron acceptor for microbial respiration • Diffusion of oxygen into water is slow • Over time, O2 in saturated soil will be used up by aerobic microbes • Facultative anaerobic bacteria (thiobacillus species) have ability to use other elements as electron acceptors in respiration • Addition of electrons "reduces" the element, i.e. Fe+3 + e- Fe+2

25. Order of Reduction • Depends on energy of the reactions O2 O-2 NO3-1 N2O or N2 (denitrification) Mn+4 Mn+2 Fe+3 Fe+2 SO4-2 S-2 (H2S) (marsh or "rotten egg" gas) HCO3-1, CO2 CH4 (methanogenesis) • Poised reactions

26. Redox Feature Formation • Reduced Fe and Mn are soluble in water • Can move by mass flow and diffusion • Reduced Fe and Mn oxidize immediately upon exposure to air and form insoluble compounds • No longer mobile. • Movement of Fe and Mn under reducing conditions form redox depletions (Fe and Mn loss) and concentrations (gain of Fe and Mn where O2 is present) form by movement of Fe • Within a horizon • Between horizons • Across the landscape

27. Fe2+ Redox Feature Formation • Depletion of Fe removes the Fe coatings that give the horizon the yellowish, brownish, or red color. • Color of the zone is that of uncoated mineral grains • Reduced Fe is also gray • Soil horizons may have reduced micro-sites and oxidized microsites at the same time

28. 105 to 150 cm; light gray (10YR 7/2) clay loam; many medium and coarse, distinct light yellowish brown (10YR 6/4) redox concentrations; weak medium subangular blocky structure; friable, sticky, slightly plastic; common fine flakes of mica; common faint clay films on faces of peds; very strongly acid; clear smooth boundary.

30. Describing Redox Features Abundance: • few - <2% of volume of layer • common - 2-20% of volume of layer • many - >20% of volume of layer Size: • fine - smaller than 2 mm • medium - 2 to 5 mm • coarse - 5 to 20 mm • very coarse - >20 mm Contrast: faint, distinct, prominent

31. Conditions for Redox Feature Formation • Saturation with water • Presence of facultative anaerobic bacteria • Organic matter as a food source • Faster reduction in surface horizons • Depletions common around dead roots • Fe • Always present except a few sandy soils • Time • 2 to 3 three weeks of saturation for Fe reduction • Shorter periods will not form redox features • Water movement and diffusion are slow • 10’s to 100’s of years to form visible redox features

32. Texture • Refers to size and relative abundance of mineral particles comprising a soil horizon or soil sample. • Soil separates - individual size groups of mineral particles.

34. Textural Triangle • A system that groups ranges of sand, silt, and clay percentages into classes • Sand, loamy sand, and sandy loam textures are modified by the dominant size class of the sand (very fine, fine, or coarse). • In FIELD: “feel” method for estimating particle size (textural classes) based on : • “gritty” vs. “smooth” • Cohesiveness (ribbon length)

35. Textural Modifiers for Rock Fragments

36. Importance of Texture • Many soil interpretations are based on texture or combination or texture and other properties • Water holding capacity (agricultural use) • Permeability, bearing capacity (urban uses) • Many pedogenic pathways and processes are inferred from texture and the distribution of texture with depth. • Weathering rates (clay formation in situ) • Translocation (clay movement with depth)

37. Soil Structure • The aggregation of primary soil particles into compound particles called peds. • Structure is described in terms of grade, size, and shape. • Grade - distinctness or degree of expression of peds. • weak • moderate • strong • Size: fine <2 mm medium 2-5 mm coarse 5-20 mm very coarse 20-76 mm extremely coarse >76 mm Shape

38. Structure Shape

39. Prismatic

40. Columnar

41. Subangular Blocky

42. Granular

43. Structure Formation • Biologic processes in surface horizons • Organic compounds from root and microbial exudates bind soil particles • Granular structure is common • Fecal pellets from earthworms, insects, and other critters. • Shrink swell in subsoil horizons • Once planes of weakness are formed by shrinkage, the soil will continue to fail along these planes • Fe and Al oxides and clay binds particles and coatings on ped surfaces enhances strength and expression of structure • Shrink/swell is driven by water content: wetting by rainfall, drying by evapotranspiration (water balance)

44. Importance of Structure • May have large impact on rates of water movement through the soil, ease of tillage, aeration, and other properties. • In soils with low shrink-swell, structure creates an extensive network of macropores that can move water rapidly under saturated conditions • Too much structure may be detrimental. • "Response of soil to management may depend as much on its structure as on its fertility."

45. Thin-section Images of Structure and Pores

46. Consistence • Compressive strength of ped (structural unit) • Resistance to deformation under pressure • Depends on moisture state • Dry soil is more difficult to deform than moist soil • Different terms for different moisture states (moist and dry). • Moist • loose, very friable, friable, firm, very firm, extremely firm • Dry • loose, soft, slightly hard, hard, very hard, extremely hard • Specimen to be tested should be 25-30 mm (1 in.) on edge. • Wet or puddled soil • stickiness - nonsticky, slightly sticky, sticky, very sticky • plasticity - nonplastic, slightly plastic, plastic, very plastic • Other terms are also used to describe brittleness, cementation, strength, smeariness, and fluidity.

47. Concentrations • Bodies that have a different color, texture, and/or composition than the matrix • Formed by concentration (accumulation) of mobile components (Fe, CaCO3, gypsum, etc). • In humid climates, related to seasonal saturation and associated mobility of Fe and Mn oxides • In drier climates, result from translocation of soluble or sparingly soluble minerals such as calcite or gypsum. • May be thin and sheet like, equidimensional, or irregular • May contrast with the surrounding matrix or may be similar • Rock fragments are not considered concentrations • Any number of properties of concentrations can be described • Most commonly described characteristics include amount, size, shape, consistence, color(s), kind and location

48. Kinds of Concentrations • Masses - soft accumulations; • Do not have clearly defined boundaries • Composition may be similar to or different from the surrounding soil • Cannot be separated from the matrix and removed as a discrete unit • Nodules and concretions • Have clearly defined boundaries and can be removed from the soil intact • Crystals • Plinthite • Fe concentration that can separated from surrounding soil, but can be broken between the fingers • Ironstone • Hardened plinthite

49. Kinds of Concentrations

50. Ped Surface Features • Coatings of unlike material • Material concentrated on ped surfaces by removal of other material • Stress features • Describe • amount, distinctness, color, texture, kind, location, and any other property that can be observed and is important. • Kinds of ped surface features: • Clay films • Clay bridges • Sand or silt coats (skeletans) • Other coatings • Stress surfaces • Slickensides